Magnetic force microscopy (MFM) is a development of the noncontact surface analysis type of atomic force microscopy (AFM). MFM is well established as a technique for determining the magnetic fields emanating from thin films, such as magnetic recording media. In an MFM system, a sharp magnetic tip is mounted on a cantilever force sensor which is placed over the surface of the magnetic specimen while the specimen is scanned by a conventional XYZ scanning stage. The magnetic forces that act on the tip from the sample cause a static deflection of the cantilever. These forces are monitored, typically by use of a laser detection system wherein the deflection of the cantilever causes a displacement of a reflected laser light beam. MFM using a magnetized iron tip is described by Martin et al., "High-resolution Magnetic Imaging of Domains in TbFe by Force Microscopy", Appl. Phys. Lett., Vol. 52, No. 3, Jan. 18, 1988, pp. 244-246. The use of silicon tips coated with a film of magnetic material, such as NiFe or CoPtCr, in MFM is described by Grutter et al., "Magnetic Force Microscopy with Batch-fabricated Force Sensors", J. Appl. Phys., Vol. 69, No. 8, Apr. 15, 1991, pp. 5883-5885.
In U.S. Pat. No. 5,436,448, a modification of this type of conventional static MFM system is shown and described in FIG. 7 of the patent. In this modified MFM system, a coil is wrapped around the cantilever having a magnetic tip on its end. Alternating current (AC) is passed through the coil which causes the magnetization of the tip to switch directions. The magnetic force data obtained with one polarity and those obtained with the other polarity are subtracted from each other to improve the resolution of the MFM system. The disadvantages of this AC-MFM system are that the magnetic field from the coil cannot be concentrated at the tip because of its physical distance from the tip, and the stray magnetic fields from the coil interact with the magnetic fields from the sample.
What is needed is an improved AC-MFM probe and AC-MFM system wherein the magnetic field can be concentrated at the probe tip and the magnetic field can be alternated at the probe tip to improve the resolution of the MFM system without stray fields from the coil affecting the magnetic sample.